Distribution of Viruses Inhabiting Heterobasidion annosum in a Pine-Dominated Forest Plot in Southern Finland.

We investigated the diversity and spatial distribution of viruses infecting strains of the root rot fungus Heterobasidion annosum collected from pine stumps at a heavily infected forest site. Four different partitiviruses were detected in 14 H. annosum isolates at the study site, constituting approximately 29% of all Heterobasidion isolates investigated (N = 48). Two of the viruses detected were new partitiviruses designated here as Heterobasidion partitivirus 16 (HetPV16) and HetPV20, and two were previously known partitiviruses: HetPV7 and HetPV13. The two new partitiviruses found, HetPV16-an1 and HetPV20-an1, shared ~70% RdRp nucleotide sequence identity with the alphapartitivirus Rosellinia necatrix partitivirus 2, and less than 40% identity with known viruses of Heterobasidion spp. HetPV7-an1 was closely similar to HetPV7-pa1 isolated earlier from Heterobasidion parviporum, supporting the view of conspecific virus pools in different Heterobasidion species. Three fungal isolates were found to be co-infected with two different partitivirus strains (HetPV7-an1 and HetPV13-an2 or HetPV16-an1 and HetPV20-an1). Different isolates representing each host clone had variable virus compositions, and virus strains occurring in more than one host clone showed minor sequence variations between clones.

Multiple virus infections on Heterobasidion sp.

Heterobasidion viruses have previously been shown to affect each other's transmission and phenotypic effects on their hosts in a complex way. In this work, Heterobasidion parviporum strains hosting five coinfecting viruses simultaneously were constructed and used as donors in transmission experiments. They showed that viruses move more frequently between the mycelia of the same species than between the mycelia of H. parviporum and Heterobasidionannosum. One of the strains was used to show that coinfection of five viruses is relatively unstable in a natural environment and analyses of the growth rate and competitive ability of Heterobasidion strains hosting various virus combinations revealed that viral effects are not additive. The results also supported the view that the transmission of the promising virocontrol agent HetPV13-an1 may be enhanced by coinfecting viruses in the donor mycelium. However, its detrimental effects may be blocked by the presence of other viruses in the same mycelium. REPOSITORIES: GenBank accession number MN058080.